1. Field of the Invention
This invention relates to an optical function element utilizing a thin film optical waveguide, and also to a method for manufacturing such optical function element.
2. Description of the Prior Art
Heretofore, research activities have been conducted in full swing as to applying a thin film type optical element using an optical waveguide to various devices such as light deflector, light modulator, spectrum analyzer, correlator, optical switch, and so forth. Such thin film type optical element functions to vary a refractive index of the optical waveguide by an external action such as acousto-optic effect, electro-optical effect, and so on, thereby modulating or deflecting light propagating in and through this optical waveguide. For the substrate to be used when forming the abovementioned optical element, there have been widely used lithium niobate (LiNbO.sub.3) crystal and lithium tantalate (LiTaO.sub.3) crystal, which are excellent in their piezoelectricity, acousto-optical effect, and electro-optical effect, and have less light propagation loss.
As a representative method of manufacturing the thin film optical waveguide using the abovementioned crystal substrate, there has been known one, in which titanium (Ti) is thermally diffused at a high temperature on the surface of the abovementioned crystal substrate to thereby form on the surface of this crystal substrate an optical waveguide layer having a refractive index which is slightly greater than that of the substrate. The thin film optical waveguide fabricated by this method, however, has various disadvantages such that it is liable to undergo optical damage, and can introduce into the waveguide path only a very small power of light. By the way, the term "optical damage" as used throughout this specification is meant "a phenomenon, in which, when the intensity of light to be input into the optical waveguide is made gradually increased, the intensity of light which propagates in and through the optical waveguide and is then taken outside becomes no longer increased owing to its scattering in proportion to the input light intensity".
As a method for fabricating the improved optical waveguide which is less liable to optical damage, there has been known one, in which the crystal substrate of LiNbO.sub.3 or LiTaO.sub.3 is heat-treated at a high temperature to diffuse lithium oxide (Li.sub.2 O) outside of the crystal substrate, thereby forming in the vicinity of the surface of the substrate a vacant interstitial layer of lithium (Li) having a refractive index slightly larger than that of the substrate.
R. L. Holman and P. J. Cressman disclose in IOC, 90, 28 April 1981 that, by the above-described external diffusion of Li.sub.2 O, a threshold value of the optical damage becomes higher than that attained by the internal diffusion of Ti. Incidentally, when the light deflector and the light modulator are to be realized in utilization of the acousto-optical effect or the electro-optical effect, an increase in efficiency of each of the abovementioned effects constitutes an important factor in the formation of the element. As a representative example of utilizing the acousto-optical effect, there is a method, in which a high frequency electric field is applied to comb-shaped electrodes formed by the photo-lithographic technique on the optical waveguide to cause elastic surface waves to be excited on the optical waveguide. It has been known, in this case, that the interaction between the elastic surface waves excited on the light waveguide and the guided light propagating in and through the optical waveguide increases as the energy distribution of guided light is confined in the vicinity of the substrate surface. (cf. C. S. Tsai, "IEEE Transaction on circuits and Systems", Vol. CAS-26, 12, 1979)
From the standpoint of the maximum utilization of the abovementioned interaction, the thickness of the optical waveguide layer (Li vacant interstitial layer) to be formed by the above-described Li.sub.2 O external diffusion method has a small change in its refractive index, on account of which the layer needs to be made as thick as 10 to 100 .mu.m or so, which is not favorable, because the energy distribution of the guided light spreads in the thickness direction of the layer. In the case of, therefore, utilizing the thin film optical waveguide as fabricated by the afore-described Li.sub.2 O external diffusion method for the light deflector, etc., it was difficult to realize a high efficiency in the resulting device.
On the other hand, as another method of fabricating the improved thin film optical waveguide with less optical damage, there has been known an ion-exchange method. In this method, the crystal substrate of LiNbO.sub.3 or LiTaO.sub.3 is subjected to a low temperature heat-treatment in a dissolved salt of tallium nitrate (TlNO.sub.3), silver nitrate (AgNO.sub.3), potassium nitrate (KNO.sub.3), and so forth, or in a weak acid such as benzoic acid (C.sub.6 H.sub.5 COOH), etc. to exchange lithium ion (Li.sup.+) in the crystal substrate for an ion species in the weak acid, such as proton (H.sup.+), etc., thereby forming the optical waveguide layer having a large difference in the refractive index (.DELTA.h.about.0.12).
While the thin film optical waveguide fabricated by the above-described ion-exchange method has an improved characteristic in its threshold value of the optical damage which is as high as several tens of times that of the thin film optical waveguide obtained by the titanium diffusion, it has a problem that the piezoelectricity and the electro-optical characteristic proper to the crystals of LiNbO.sub.3 and LiTaO.sub.3 become poor by the ion-exchange process, with the consequence that, when it is used for the light deflector, for example, the guided light inevitably lowers its diffraction efficiency.